The molecular organization of paired helical filaments, amyloid plaque cores, and cerebrovascular amyloid isolated from Alzheimer's disease (AD) brain will be studied using X-ray diffraction and electron microscopy. High-resolution X=ray patterns will be obtained from isolates of the abnormal fibrous assemblies for comparison with patterns from in vitro assemblies of synthetic polypeptides having sequence homologies with the AD amyloid beta-protein. To obtain patterns that are suitable for such comparisons, the assemblies will be oriented by exposure of pellets or concentrated solutions to an external magnetic field of 2 Tesla. The magnetically-induced orientation of similar filamentous structures (as diverse as filamentous bacteriophage, whole retinal rods, nucleic acids, and sickle cell hemoglobin fibers) has been previously shown to substantially enhance the quality of the diffraction data. The X-ray scatter from our various fibrous assemblies will also be enhanced by altering the electron density of the bathing medium, using negative stains, and labeling reactive amino acid residues with heavy metals. Appropriately contrasted and well-oriented pellets will allow the detection of weak scatter that arises from the packing of the structural units within and along the fibres. Accurate determination of the spacings and of the integral widths of the X-ray reflections will provide a measure of the sizes of the diffracting regions and the lengths of the polypeptide chains constituting the structure. Determining the dimensions and spatial organization of the structural units that comprise paired helical filaments, AD amyloids and in vitro assemblies will illuminate our understanding of their formation, interrelationships, and stability, and will provide insight on how they might be related to or derive from normally occurring cytoskeletal components in the neuron. Of more general interest, our findings will also be relevant to the systemic amyloidoses, and to the problem of protein folding.